Optical networks frequently use fixed wavelength laser sources. However, widely tunable lasers are advantageous over fixed lasers in this context. For example, an eighty channel network with five regeneration points requires almost five hundred fixed wavelength lasers. Each of these fixed wavelength lasers requires a backup, which means there are approximately five hundred backup network cards sitting idle in inventory at a given time. Since each of these cards can cost between $10,000 and $50,000, this is an expensive proposition. If widely tunable lasers are used in place of the fixed lasers, the number of backup cards required by this network is reduced by at least the channel count, which results in a substantial cost savings.
In addition to these financial savings, employing widely tunable lasers instead of fixed lasers has other advantages. For example, tunable sources permit flexible, more responsive provisioning of bandwidth, thereby simplifying network planning and expansion of the network as a whole. Widely tunable sources also enable the network provider to dynamically or statically assign consumers their own wavelength channel(s). Moreover, tunable light sources can be used in optical networks to perform routing on a wavelength basis.
A prior art tunable laser 10 is shown in FIG. 1. This conventional external cavity laser 10 includes a semiconductor laser 12 and two Bragg gratings 14. Each of the Bragg gratings 14 is coupled to an end of the laser 12 via a passive waveguide 16. Each of the gratings 14 functions as an end mirror, and at least one of the gratings 14 (e.g., the grating 14 at the left side of FIG. 1) reflects some light and passes some light to provide the laser output.
Example reflection spectra for the Bragg gratings 14 are shown in FIG. 2. Each of the illustrated reflection spectra includes a set of high reflectivity peaks. Since, as shown in FIG. 2, each of the Bragg gratings 14 has a different period, the positions of the peaks associated with the gratings 14 are largely out of alignment. However, one pair of the peaks is in alignment (e.g., the peaks at approximately 1.31 μm (micro meters)). This overlap determines the lasing wavelength since light is being coherently reflected back and forth through the gain chip 12 at this wavelength. Changing the index of refraction of either of the gratings 14 will cause the reflection peaks associated with that grating to shift. Therefore, changing the index of refraction of one or both of the gratings 14 will cause the lasing wavelength to hop from one successive peak of the reflection spectrum of the other grating 14 to the next (see FIG. 3 where the lasing wavelength has shifted to about 1.328 μm).
Significantly, as can be seen by comparing FIGS. 2 and 3, a relatively small shift in the reflection spectrum of one of the gratings 14 results in a relatively large shift in the lasing wavelength due to the vernier-like effect between the spectra of the gratings 14. Thus, changing the index of refraction of one or both of the gratings 14 permits tuning of the laser over a wide range of wavelengths.
Tuning of the laser can be achieved by adjusting the index of refraction of one grating or by adjusting the indices of refraction of both gratings 14 simultaneously. Optionally, the laser may incorporate a phase section to achieve substantially continuous tuning without hoping between cavity modes.
One disadvantage of leveraging the vernier-like effect of two Bragg gratings is the packaging difficulty. In particular, each of the Bragg gratings 14 must be coupled to an end of the laser gain chip 12 as shown in FIG. 1. Additional packaging is then needed to couple the final laser 10 to an output fiber (not shown).
Tunable laser sources have also been produced by coupling an anti-reflection (AR) coated Fabry-Perot laser diode to an external cavity. The laser diode provides the gain. The external cavity provides wavelength tuning. The wavelength selective external cavity may include gratings, etalons or arrayed waveguides (AWG's) in order to achieve tuning.